This invention relates generally to digital radio communications systems, and more particularly to techniques employed by digital radio communications systems to improve co-channel reuse.
Early systems used Carrier Sense Multiple Access (CSMA) and no frequency re-use. A variation called Busy Tone Multiple Access (BTMA) has been shown more efficient. Most applications today would employ the digital equivalent of BTMA, which might be either a repetitive bit or a repetitive field in a transmitted bitstream instead of a busy tone for inhibiting access, thus permitting the busy tone channel to carry other data. Either method could be more generally described as Busy Signal Multiple Access (BSMA).
U.S. domestic cellular radiotelephone systems, in accordance with EIA Standard IS-3-D, employ BSMA and frequency re-use. Each cell is assigned one of a plurality of Forward Control Channels designated as Access Channels, paired in radio frequency with Reverse Control Channels, and reused in such a way as to minimize interference between cells. Both the subscriber terminals (conventional cellular radiotelephones) and the base stations operate full-duplex. Repetitive bits called B/I bits positioned in the Forward Control Channel message stream provide the Busy/Idle indication. The subscriber terminal, prior to seizing an Access Channel, must first select which of the plurality it is receiving the best. It then determines its B/I status; if busy, it enters a random time-out to re-try, but if not busy, the terminal starts sending a message on the corresponding Reverse Control Channel. If the channel becomes busy before the first 56 bits of this message is sent, the terminal must immediately turn off its transmitter. Sometimes the Reverse Control Channel in a first cell receives a message from a remote subscriber terminal, the FM receiver of which is undesirably captured by a second cell using the same frequency pair. When this happens, the Forward Control Channel of the first cell will raise its busy indication, and may actually attempt to exchange messages with the subscriber station, all for naught, because the subscriber terminal's receiver is captured by the second, and the protocol and control means ultimately protects the system. Still, this effect reduces the capacity of signalling circuits.
In Motorola's radio data terminal systems of the MDC-4800 family, a plurality of full-duplex base stations, all assigned the same radio frequency pair, provide a service for a multiple of half-duplex radio terminals in a large geographic area. All base stations are connected to and controlled by a Network Control Processor (NCP), from which they get all their control instructions and transmitted data packets, and to which they send all their received data packets (along with pertinent information such as received signal strength) and all their status information. This NCP also has numerous connections to land terminals and computers, such as dedicated packet network connections, modems connected into leased landline circuits, and auto-dial and auto-answer land network modem circuits. Unlike cellular systems, the base stations are located to provide extensive coverage overlap. The NCP manages the base stations so that both overhead messages and land (or terminal) originated pages and messages toward the terminals are broadcast first by one group of (non-overlapping coverage) base stations and then another, until either the addressee responds, or the entire service area has been blanketed. In normal operation, a number of the base stations transmit simultaneously, and all base stations transmit at least some information every several minutes. Repetitive bits in the base station's transmitted stream are reserved for BSMA idle/busy indication; whenever a base station that is transmitting detects the presence of signal above a prescribed threshold from one of the terminals, it indicates the busy state. Terminals wishing to transmit, if receiving busy state, will execute a random time-out to re-try. Base stations able to receive any packet and relay that packet, along with the received signal strength, to the NCP. The NCP, in turn, eliminates duplications and forwards the packets to the addressees. The NCP also decides which base station will reply with what packet. Because of the extensive overlap, terminals are often unnecessarily inhibited when another terminal's transmission raises the busy condition on several nearby and sometimes distant base stations, thus limiting the amount of frequency re-use.
In digital cellular telephone systems, a plurality of frequencies and a high degree of frequency re-use is anticipated. In one scheme described in U.S. Pat. No. 4,876,740, the synchronization word (called SAT/SYNC) is also used as a means of conveying a limited amount of base station site information to the subscriber terminal. The terminal selects the base station of its choice and responds with that station's SAT/SYNC, and all base stations tuned to the same radio frequency with different SAT/SYNC sequences, located at nominal re-use distances, ignore the terminal, since their synchronizing correlators are set only to receive packets in which the preamble is their own SAT/SYNC. However, for a large radio data communications network with but one heavily overlapped radio frequency pair, it is desirable for base stations to receive any message strong enough to receive it error-free; SAT/SYNC does not provide enough useful variations with good synchronizing properties for use as a base station identification number.
Therefore, it is one object of the present invention to provide, in a BSMA radio data communications system employing a plurality of full-duplex base stations assigned the same radio frequency pair, a method of effectively using, at a base station, combinations of the received signal strength of a transmitting radio terminal, that radio terminal's preference of base station and that base station's signal strength at that radio terminal in order to restrict the area of coverage over which busy signalling is broadcast, thus improving frequency re-use.
It is another object to provide, in such a communications system, a method of effectively using, at a base station, the received signal strength of a transmitting radio terminal and that terminal's preference of base station, and at a receiving radio terminal's received signal strength in order to restrict the defined busy area, thus improving frequency re-use.